Concern for the environment as well as increasingly stringent regulation of air quality, pollution and hazardous waste has created a need for alternatives to the use of petroleum and petroleum-based products in ink compositions. The primary focus has been on the development of water- and soy-based printing inks. The major reason for using water-based ink compositions is that because organic solvents are not present except in very small quantities, potential problems such as toxicity, flammability and other hazards are minimized.
Traditional water-based inks are based on resin systems which are dispersed in water. Several references describe inks which are based on aqueous vehicles. U.S. Pat. No. 5,106,417 to Hauser et al., for example, describes low viscosity aqueous printing ink compositions based on polyacrylic resins containing carboxyl groups, and which also contain a humectant, pigment and, optionally, a water-dilutable organic solvent. U.S. Pat. No. 5,166,245 to Zuraw et al. describes water-based printing inks made with modified rosin grinding resins. U.S. Pat. No. 5,242,489 to Schwarz, Jr., et al., describes an ink composition containing a polymeric additive in addition to a colorant and an aqueous liquid vehicle. U.S. Pat. No. 5,244,496 to Easton et al. describes a water-fast ink composition containing an amine, an acrylic emulsion resin binder, a triamine dye, and water.
The major drawback of the currently available water-based inks, however, is their generally poor water fastness, which limits their utility.
The present invention is addressed to the aforementioned limitation in the art, and provides a water-based ink composition which is also water-fast. That is, the ink compositions are prepared in water, but after printing and drying become insoluble in water and, depending on the formulation, are also insoluble in alkaline solutions, detergents, and common organic solvents such as alcohols, acetone and toluene.
The novel ink compositions are advantageous in other ways as well. In contrast to many ink compositions of the prior art, the compositions of the invention may be used on a wide variety of substrates, dry quickly after application, and tend not to bleed, smear or rub after drying. The binder materials and colorant are selected so that the colorant is entrapped by the binder matrix, substantially eliminating the possibility that the colorant will leach. In addition, the present ink compositions are generally low in viscosity (and thus relatively easy to work with), display good soak resistance, are generally stable at elevated temperatures, and have a relatively long shelf life, typically on the order of a year or more. It must also be emphasized that the ink compositions may be readily tailored with respect to (1) viscosity, surface tension, dielectric constant and color density by varying the amount or type of components used, and (2) end use, i.e., as continuous jet printing inks, drop-on-demand inks, or the like.